The present invention relates to method and apparatus for plasma cutting a workpiece. More particularly, the invention relates to such method and apparatus in which a surface roughness of the cut facet of the workpiece is improved by controlling the frequency of a pulsating current to be superimposed on a D.C. cutting current in accordance with the moving speed of a cutting torch.
Plasma cut is used for cutting a workpiece, in which plasma arc energy generated from a cutting torch (hereinafter referred simply to as "torch") is locally concentrated onto the workpiece, thereby melting the workpiece. While the plasma cut is advantageous in that the cutting speed is fast and a cutting deformation is unlikely to occur, it is disadvantageous in that complicated surface irregularities are formed on the cut facet of the workpiece due to distorted flow of plasma gas stream and/or shift of the anode point in the workpiece. It is further disadvantageous in that a large amount of melted metal resulting from the plasma arc adhere to the workpiece as dross.
Japanese Patent Laid-Open Publication No. 61-262464 discloses a technique for reducing the adhesion of the dross, in which a 400 Hz pulsating current which frequency is relevant in removing the dross is superimposed on a D.C. cutting current (base current).
Generally, in the cases where the cut facet of the workpiece is formed with irregularities and thus the surface roughness is extraordinarily low and where the dross is adhered to the cut facet thereof, the cut facets are ground by a grinder to thus smoothen the cut facet.
The present inventors have found that the cut facet irregularities is suppressed to a minute level if a pulsating current having a relatively low range frequency, e.g. 100 to 500 Hz, is superimposed on a D.C. current to supply the resulting current to the torch.
In the plasma cutting apparatus disclosed in the above-mentioned publication, since a pulsating current of a constant 400 Hz is superimposed on the cutting current, adhesion of the dross can effectively be prevented and the surface roughness may be improved to some extent. However, when the cutting speed is varied depending on the kind of the material of the workpiece, the thickness thereof, and the cutting path, such as curves including arcuate or segmental curves, the number of pulses of the pulsating current allocated to a unit length of the cut length is not constant, so that the surface roughness cannot be uniformly improved all over the cut facet.
High frequency pulsating current is not effective in improving the surface roughness as the plasma arc does not respond to the frequency of the pulsating current.